The present invention relates to a method of manufacturing a stamper for producing optical discs, comprising the application of a photoresist to a stamper plate and the structuring of the applied photoresist film, wherein said structuring comprises the successive exposure, development and heating of said photoresist film, wherein the developed photoresist film is subjected to an additional exposure step in the deep UV range prior to the final heat treatment. The present invention furthermore relates to a stamper for producing optical discs and to optical discs obtained by means of such a stamper.
A method of this kind is known from Dutch patent application no. 9400225 filed in the name of the present applicant. According to the method of manufacturing a stamper described therein, a photoresist film is applied to a stamper plate, after which the applied photoresist film is structured. The structuring of the applied photoresist film comprises the successive selective exposure and development of the applied photoresist film, wherein the structuring of the applied photoresist film furthermore comprises, in particular as an additional step, the heating of the selectively exposed photoresist film. In addition to that, the structuring of the applied photoresist film furthermore comprises the integral exposure of the photoresist film prior to developing the photoresist film. The successive steps of selective exposure, heating and integral exposure prior to the developing step are also known as xe2x80x9cimage reversal processxe2x80x9d to those skilled in this field of the art. According to the example carried out in said Dutch patent application, a negative photoresist was coated, by means of spin-on deposition, onto a blank 300 xcexcm thick nickel shell, which was glued on a standard 8-inch CD glass substrate for this experiment. After dry-spinning of the nickel shell/glass substrate assembly at a constant temperature of 80xc2x0 C., the applied negative photoresist film was dried. After cooling, the assembly was exposed by means of a master registration system (MRS) at a wavelength of 459 nm. Directly after said exposure, the assembly was heated and subsequently cooled, after which integral exposure took place for 4 minutes. Thus, the non-exposed parts of the negative photoresist film applied to the nickel shell were exposed as yet. The assembly was subsequently developed and heated again, in the same manner as after the exposure, for the purpose of fixating the structure of the photoresist posts, after which cooling took place, with this difference that the oven temperature set for heating was now 140xc2x0 C.
In practice, however, this latter heat treatment, also called hardbake, has been carried out at a temperature of 200xc2x0 C. the last few years. A drawback of this relatively high temperature of the final heat treatment is the fact that the so-called photoresist posts formed after the developing step lose their structure or geometry in an uncontrolled manner, as a result of which they will flow out more or less. Since the geometry of the photoresist posts will change as a result of the necessary heat treatment at a high temperature, the obtained stamper often does not meet the precise specifications that are required for producing optical discs, which is undesirable.
Consequently, it is an object of the present invention to provide a method of manufacturing a stamper as referred to in the introduction, which method eliminates the aforesaid problems.
Another object of the present invention is to provide a method of manufacturing a stamper as referred to in the introduction, which method allows or effects a certain amount of flowing of the photoresist posts, making it possible to control the desired post geometry which is required for mass replication.
Surprisingly, the above objectives have been accomplished by the present inventors by providing a method as referred to in the introduction, which is characterized in that the effective dose of said additional exposure step ranges between 4xc2x710xe2x88x924 and 5xc2x710xe2x88x922 J/cm2 with a wavelength of 200-320 nm.
According to the present invention, the effective dose or energy density of the additional exposure step thus ranges between 4xc2x710xe2x88x924 and 5xc2x710xe2x88x922 J/cm2, preferably between 8xc2x710xe2x88x924 and 1.2xc2x710xe2x88x922 J/cm2. If the effective dose of the additional exposure step according to the present invention is less than 4xc2x710xe2x88x924 J/cm2, no suffficient crosslinking effect will occur in the photoresist film. If the effective dose of the additional exposure step according to the present invention is more than 5xc2x710xe2x88x922 J/cm2, no significant crosslinking effect will occur in the photoresist film. According to the present invention, the term effective dose is understood to mean the product of the dose and the sensitivity of the exposed material at the wavelength used. For novolac resin, for example, the maximum sensitivity is at a value of xcex=250 nm, as a result of which the energy dose being used at a wavelength of 250 nm substantially corresponds with the effective dose. It is apparent that a wavelength other than 250 nm will require a higher energy dose in order to obtain the same effective dose and thus the same effect as when a wavelength of 250 nm is used.
Although it is known from the article xe2x80x9cDeep UV hardening of positive photoresist patternsxe2x80x9d, Journal of the Electrochemical Society, part 29, No. 6, June 1982, pages 1379-1381 in the name of Allen R. et al. to reduce the thermal deformation, in particular the flowing of photoresist patterns, during treatments at a high temperature of 180xc2x0 C. for 30 minutes by exposing the photoresist in the deep UV range by means of a low-pressure mercury vapour lamp, said article discloses an exposure of for example 8 mW/cm2 for 20 minutes, which corresponds with an energy dose of 9,6 J/cm2. Such a dose in accordance with the article of Allen R. et al. is substantially higher than the effective dose in the range of 4xc2x710xe2x88x924-5xc2x710xe2x88x922 J/cm2 and a wavelength of 200-320 nm which the present invention employs, which dose makes it possible to control said flowing, thus making it possible to control the geometry of the photoresist posts being formed.
The use of an additional exposure step before the hardbake step is also known from the articles xe2x80x9cUV-hardening of Resist Patternsxe2x80x9d, IBM Technical Disclosure Bulletin, part 24, No. 3, August 1981, and from the article xe2x80x9cPhotoresist Stabilization Systemxe2x80x9d, Solid State Technology, part 27, No. 7, June 1984, Washington, USA, but neither article discloses any exposure energy values. Said use corresponds with the aforesaid article by Allen, R et al., however, in particular IC lithography, so that it is likely that the energy values being employed will also be the same as in said article, that is, considerably higher than the values employed according to the present invention.
Such an additional exposure step according to the present invention with a very effective dose will probably result in oxidation reactions in the photoresist, in particular in the outer layers of the photoresist, which oxidation reactions lead to curing and a higher glass transition temperature. As a result of that, the structure of the photoresist posts formed during the developing step will not be adversely affected by the subsequent heat treatment at a high temperature, and it will probably be controllable. However, the invention is not limited by this assumption regarding the oxidation reaction that will probably occur.
The additional exposure step according to the invention, which is to be carried out prior to the final heat treatment, is preferably carried out in the range in which the photochemical curing of the photoresist takes place, that is, in the range in which absorption of the electromagnetic radiation takes place, in particular in the range of 200-320 nm, more in particular in the range of 240-260 nm.
The selection of the wavelength range to be used for the additional exposure step has an influence on the curing process of the photoresist. Although the scientific explanation as regards the relation between the wavelength and the curing of the photoresist is not quite clear to those skilled in this field of the art, the present inventors assume that the following applies. It should be understood thereby that the present inventors are by no means limited by such an explanation. In the long wavelength range, for example 300-320 nm, the penetration depth of the UV radiation is great, and complete curing of the photoresist will take place. When a wavelength of 240-260 nm is used, the absorption by the photoresist will be large, as a result of which the penetration depth of the UV radiation will be small and only curing of the outermost layer of the photoresist will take place, therefore. In this latter embodiment, the combination of the very effective dose of the additional exposure step, exposure in a specified wavelength range and subsequent heating makes it possible to influence the post geometry of the photoresist posts. If the absorption peak of the photoresist being used ranges between 240 and 260 nm, which applies to novolac resins, an additional exposure step outside the aforesaid range will not effect sufficient absorption. Inadequate curing of the photoresist film will result, which means that the structure of the photoresist posts will be affected during the subsequent heat treatment at a high temperature, with flowing out taking place to a larger or smaller degree, which is desirable in certain embodiments.
The exposure time of the additional exposure step according to the present invention is preferably 1-125 seconds, more preferably 2-30 seconds. Experimental data have shown that an exposure time of less than 1 second will probably lead to insufficient oxidation reactions in the photoresist film and thus to insufficient curing. An exposure time of more than 125 seconds does not have an additional effect on the oxidation reactions induced in the photoresist film. In addition to that, an exposure time of more than 125 seconds is undesirable because the photoresist posts, which mainly consist of organic material, will be slowly affected as a result of the ozone production, causing them to evaporate, mainly in the form of H2O and CO2. Moreover, when such a long exposure time, the heat production caused by the additional exposure step according to the present invention will be high, as a result of which the temperature may rise to above the glass transition temperature, which will have an adverse effect on the geometry of the photoresist posts as yet.
In a preferred embodiment of the method according to the invention, the additional exposure step is carried out under rotation. Since the stamper is rotated during said additional exposure, a crosslinking effect occurs in the photoresist film over the entire area of the stamper, as a result of which a satisfactory evenness is achieved.
In addition to that, it is preferred to carried out the additional exposure step under heating. As a result of said heating, the crosslinking effect in the photoresist film will take place more quickly, as a result of which the exposure time of the additional exposure step according to the present invention can be reduced, which results in a higher production speed.
With the method according to the present invention, a negative photoresist is preferably used for applying the photoresist film. Exposed areas of a film of a negative photoresist remain after developing, whilst the non-exposed areas are removed as a result of the developing step. The method according to the present invention is not limited to the use of a special negative photoresist, however. More in particular, the method according to the present invention is suitable for influencing the post geometry of photoresist posts, irrespective of the special origin of the photoresist being used, that is, a positive photoresist or a negative photoresist. By applying a negative photoresist to a stamper plate, which might in principle be placed directly in the injection moulding unit, and structuring said negative photoresist in accordance with the proposed method, both the metallisation step and the electroplating step may be skipped.
In a preferred embodiment of the method according to the invention, the exposure step, which is not the additional exposure step according to the present invention, is carried out in two steps, that is, a first selective exposure step and a second integral exposure step. By selectively exposing the negative photoresist, an acid is formed in the exposed areas. The heating of the negative photoresist following said selective exposure step will lead to crosslinking of said photoresist in the exposed areas under catalysis of said acid. As a result of the integral exposure step, an acid is formed in the non-exposed areas as well, which acid causes the areas which have not been selectively exposed, and which are thus not crosslinked, to dissolve sooner during the developing step. As a result of the heating step following the developing step, the selectively exposed and thus crosslinked areas are strengthened by further crosslinking of polymer chains, whereby such a heating step is according to the present invention preceded by an additional exposure step as already described above.
The present invention furthermore relates to a stamper for producing optical discs, which stamper is characterized in that it has been obtained by using the method according to the present invention.
The present invention furthermore relates to an optical disc, which disc is characterized in that it has been obtained by using the stamper according to the present invention.